101 favorites you guys And I made this on a whim at 2 in the morning ^^" Lol I'm so glad it's actually helping people >w<
Anyways, this is how i do it. I pretty much took bits of advice i scraped from across the internet to develop my own method to doing this... it's probably not the only way to get transparent backgrounds, but it's the way I do it, so... it works for me^^'
If you finish a project and go 'oh crap, the parts I meant to be white in the picture turned transparent too!', no worries, just place a new layer between the transparent layer and your art and fill the white spaces in.
Also, since SAI detects the pixel opacity, it's a good idea to color in white behind parts of your image that aren't completely saturated with color.
Aaaaand you don't have to flatten your image first. But a few times, when i finished this process, there were odd white chunks that I couldn't get to go away, and when I flattened the image and THEN went through this process, it resolved itself perfectly.
If you have any more questions, I can try to help, but I am by no means an expert. Just another learner who found a neat trick i wanna share^^
Hope this helps~<3